The term "pillared clays" as used herein refers to pillared natural and synthetic sheet-type (lamellar) silicates, and may properly be regarded as pillared derivatives of such lamellar precursors. The relationship of the structure and properties of the precursor materials to the structure and properties of the pillared derivatives are perhaps best shown by briefly considering the natural smectite clays and their pillared derivatives.
The natural smectite clay minerals have a structure consisting of superposed lamellas separated from each other by a layer of hydrated cations. Each of the lamellas is a two-dimensional polymeric oxyanion formed by two superficial layers consisting of tetrahedral sites bonded to a central layer of octahedral sites. The individual lamellas are about 9.6 A.U. thick. The 2:1 relation between the tetrahedral and octahedral layers within a lamella is characteristic of the smectite clays. Clays of the smectite type include montmorillonite, beidelite, nontronite, and others. The smectite clays have in common the property that they can undergo ion exchange, for example with acids, i.e. the intercalated cations are mobile. They also have the ability to intercalate metal complexes, organic species, and solvent such as water with increase in the interlayer distance. In some cases, such as with sodium montmorillonite immersed in water, the osmotic swelling leads to such large increase in the interlayer distance with concommitant decrease of interlamellar bonding force as to delaminate the clay. Such peptization is reversible.
It is evident from the foregoing description that the interlamellar distance in the natural smectites is variable and depends to a very large extent on the presence of liquid water. At 100.degree. C. to 200.degree. C., the interlamellar space decreases to about the thickness of a monolayer of water. Thus, the precursor materials do not have the well defined and fixed pore volume characteristic of inorganic sorbents and porous catalysts. The concept of pillaring smectite clays to create a porous network appears to have been first described by Barrer and MacLeod in Trans. Farad. Soc. Vol. 51, p. 1290 (1955), when they used tetraalkylammonium ions to limit the distance to which lamellas could be brought together. Since that time other pillaring agents have been proposed, including metal chelate complexes, and, most recently, polynuclear hydroxy metal cations. The use of oligomeric metal cations such as hydroxy aluminum and hydroxy zirconium cations can provide pillared phases with interlayer free spacings in the range of 5 to 20 A, and which are thermally stable above 500.degree. C. in the absence of water vapor. Such materials are of interest as catalysts and catalyst supports for processing petroleum streams, and as sorbents. For a fuller description of pillared (and other modified) clay catalysts, the reader is referred to a publication by T. J. Pinnavaia in Science, Vol. 220, No. 4595, pp. 365-371 (Apr. 22, 1983), the entire content of which is incorporated herein by reference for background purposes.
The smectite clays are one example of a class of lamellar clays that lends itself to pillaring. Other types of expandable sheet-structure clay type minerals that also lend themselves to pillaring include vermiculite, nontronite, saponite, hectorite, biotite, magadiite, sauconite, bowlingite, and mixed-layer type minerals such as illite-montmorillonite, rectorite, allevardite, hydromicas, and others.
Pillared clays in general are recognizable because the basal plane distance (interlamellar spacing) remains essentially unchanged on progressive removal of water at elevated temperature. Basal plane distance is readily measured by X-ray diffraction, as is known to those skilled in the art. Furthermore, an outgassed pillared clay, free of sorbate, has a significant sorption capacity for organic compounds such as benzene. In general, a calcined clay well-pillared with thermally stable inorganic polymer will have an interlayer spacing of at least about 6 A.U. (Angstrom Units), and may range up to about 20 A.U. It will have a nitrogen BET surface area greater than about 100 m.sup.2 /g, and a nitrogen pore volume of about 0.1 to about 0.6 cc/g. U.S. Pat. No. 4,176,090 to Vaughan et al. and U.S. Pat. No. 4,238,364 to Shabtai describe such pillared clays and their utilities. These are incorporated herein by reference as if fully set forth.
As will be illustrated by example hereinbelow, clays pillared, for example with oligomeric hydroxy aluminum and hydroxy zirconium cations, have a serious deficiency. In the presence of steam at elevated temperature the rigid three-dimensional structure at least partially collapses with loss of pore volume. This limits the potential usefulness of such structures to catalytic applications in which no water is present, either in the feed or in the product, and to applications which do not require periodic regeneration of the catalyst by burning off coke deposits since such deposits contain hydrogen and burning generates steam. It also limits the use of such structures as sorbents to situations in which desporption with regeneration of active sorbent is conducted in the absence of steam.
It is an object of this invention to provide a method for extending the useful life of pillared clay catalysts and sorbents. It is a further object of this invention to provide a method whereby spent pillared clay catalysts are regenerated without loss of catalytic activity. It is a further object to provide a method for steam regeneration of a pillared clay sorbent without loss of sorption capacity.